linux-kernel - Re: [PATCH] PM / QoS: Fix device resume latency PM QoS

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Message-ID: <2465945.6cfLQGiipl@aspire.rjw.lan>
Date:   Tue, 24 Oct 2017 10:49:18 +0200
From:   "Rafael J. Wysocki" <rjw@...ysocki.net>
To:     ramesh.thomas@...el.com
Cc:     Linux PM <linux-pm@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Reinette Chatre <reinette.chatre@...el.com>,
        Alex Shi <alex.shi@...aro.org>,
        Ulf Hansson <ulf.hansson@...aro.org>
Subject: Re: [PATCH] PM / QoS: Fix device resume latency PM QoS

On Tuesday, October 24, 2017 7:54:09 AM CEST Ramesh Thomas wrote:
> On 2017-10-20 at 13:27:34 +0200, Rafael J. Wysocki wrote:
> > From: Rafael J. Wysocki <rafael.j.wysocki@...el.com>
> > 
> >  
> >  static ssize_t pm_qos_resume_latency_store(struct device *dev,
> > @@ -228,11 +235,19 @@ static ssize_t pm_qos_resume_latency_sto
> >  	s32 value;
> >  	int ret;
> >  
> > -	if (kstrtos32(buf, 0, &value))
> > -		return -EINVAL;
> > +	if (!kstrtos32(buf, 0, &value)) {
> > +		/*
> > +		 * Prevent users from writing negative or "no constraint" values
> > +		 * directly.
> > +		 */
> > +		if (value < 0 || value == PM_QOS_RESUME_LATENCY_NO_CONSTRAINT)
> > +			return -EINVAL;
> >  
> > -	if (value < 0)
> > -		return -EINVAL;
> > +		if (value == 0)
> > +			value = PM_QOS_RESUME_LATENCY_NO_CONSTRAINT;
> > +	} else if (!strcmp(buf, "n/a") || !strcmp(buf, "n/a\n")) {
> 
> Can the 2 checks for "n/a" be combined by checking first 3 characters?

No, because "n/asomething" would then match too.

> > +		value = 0;
> > +	}
> 
> Should there be a check for kstrtos32 failure and return -EINVAL?

No, but there should be an "else" branch here.

> >  
> >  	ret = dev_pm_qos_update_request(dev->power.qos->resume_latency_req,
> >  					value);
> 
> 
> > Index: linux-pm/drivers/base/power/domain_governor.c
> > ===================================================================
> > --- linux-pm.orig/drivers/base/power/domain_governor.c
> > +++ linux-pm/drivers/base/power/domain_governor.c
> > @@ -14,23 +14,20 @@
> >  static int dev_update_qos_constraint(struct device *dev, void *data)
> >  {
> >  	s64 *constraint_ns_p = data;
> > -	s32 constraint_ns = -1;
> > +	s64 constraint_ns = -1;
> >  
> >  	if (dev->power.subsys_data && dev->power.subsys_data->domain_data)
> >  		constraint_ns = dev_gpd_data(dev)->td.effective_constraint_ns;
> >  
> > -	if (constraint_ns < 0) {
> > +	if (constraint_ns < 0)
> >  		constraint_ns = dev_pm_qos_read_value(dev);
> > -		constraint_ns *= NSEC_PER_USEC;
> > -	}
> > -	if (constraint_ns == 0)
> > +
> > +	if (constraint_ns == PM_QOS_RESUME_LATENCY_NO_CONSTRAINT)
> >  		return 0;
> >  
> > -	/*
> > -	 * constraint_ns cannot be negative here, because the device has been
> > -	 * suspended.
> > -	 */
> > -	if (constraint_ns < *constraint_ns_p || *constraint_ns_p == 0)
> > +	constraint_ns *= NSEC_PER_USEC;
> > +
> > +	if (constraint_ns < *constraint_ns_p || *constraint_ns_p < 0)
> >  		*constraint_ns_p = constraint_ns;
> >  
> >  	return 0;
> > @@ -63,10 +60,14 @@ static bool default_suspend_ok(struct de
> >  
> >  	spin_unlock_irqrestore(&dev->power.lock, flags);
> >  
> > -	if (constraint_ns < 0)
> > +	if (constraint_ns == 0)
> >  		return false;
> >  
> > -	constraint_ns *= NSEC_PER_USEC;
> > +	if (constraint_ns == PM_QOS_RESUME_LATENCY_NO_CONSTRAINT)
> > +		constraint_ns = -1;
> > +	else
> > +		constraint_ns *= NSEC_PER_USEC;
> > +
> >  	/*
> >  	 * We can walk the children without any additional locking, because
> >  	 * they all have been suspended at this point and their
> > @@ -76,14 +77,19 @@ static bool default_suspend_ok(struct de
> >  		device_for_each_child(dev, &constraint_ns,
> >  				      dev_update_qos_constraint);
> >  
> > -	if (constraint_ns > 0) {
> > -		constraint_ns -= td->suspend_latency_ns +
> > -				td->resume_latency_ns;
> > -		if (constraint_ns == 0)
> > -			return false;
> > +	if (constraint_ns < 0) {
> > +		/* The children have no constraints. */
> > +		td->effective_constraint_ns = PM_QOS_RESUME_LATENCY_NO_CONSTRAINT;
> > +		td->cached_suspend_ok = true;
> > +	} else {
> > +		constraint_ns -= td->suspend_latency_ns + td->resume_latency_ns;
> > +		if (constraint_ns > 0) {
> > +			td->effective_constraint_ns = constraint_ns;
> > +			td->cached_suspend_ok = true;
> > +		} else {
> > +			td->effective_constraint_ns = 0;
> 
> Previously effective_constraint_ns was left as -1 if constraint_ns becomes 0
> Not sure if this change is intentional.

Yes, it is.

> I think at dev_update_qos_constraint, this can cause to skip call to
> dev_pm_qos_read_value. 

I need to double check that.

Thanks,
Rafael